In a twin-drum strip caster, basic properties such as heat resistance to withstand a molten steel temperature of about 1,600° C., thermal shock resistance to withstand a temperature difference of about 400° C., less thermal deformation (less thermal expansion coefficient), less wettability with molten steel (better detachability of a coagulation), and excellent wear resistance are required of a material used for side dams which interpose cooling drums from the both sides and form a molten steel pool (refer to Japanese Unexamined Patent Publication No. S62-166054).
At the present time, when longer service life of continuous casting tends to be required, in addition to the basic properties, excellent erosion resistance is also required of side dams, since they erode at the portions where both the cooling drums and molten steel contact them (the portions of the molten steel side along the slide planes facing the cooling drums), and if the continuous casting goes on for an extended period of time, the erosion increases and the service life of the side dams shortens (refer to Japanese Unexamined Patent Publication No. H7-68354).
However, there has been to date no ceramic plate material available which satisfies all of those requirements, and the plates have been formed by sticking or laying together different ceramic materials each of which satisfies some of the required properties at the portions where prescribed properties are required (refer to Japanese Unexamined Patent Publication Nos. H3-207554, H7-60411, etc.).
Those plates demonstrate excellent performance as side dams in the case of a short-time continuous casting operation, but are not adequate for a long-time continuous casting operation, since the performance is insufficient due to their structural restrictions.
As a ceramic plate material, boron nitride (BN) has mainly been used in spite of its high cost, since it has the advantages of satisfying many of the above required properties (heat resistance, thermal shock resistance, less thermal deformation, less wettability with molten steel, etc.), not causing cracks during actual casting, easily fitting with the drum end faces at the initial stage because of its softness, thus forming negligible clearances between the side dams and the drum end faces, and thus preventing molten steel intrusion.
However, since BN is soft, it therefore has the disadvantages in that it is abraded by the cooling drums in a short period of time and cannot withstanding a long-time continuous casting operation.
Therefore, to make up for the disadvantages of BN, some ceramic plate materials comprehensively having the properties required of side dams have been developed by combining other ceramics, for example, silicon nitride (Si3N4) and aluminum nitride (AlN), with BN.
As an example, the above-mentioned Japanese Unexamined Patent Publication No. H7-60441 discloses a ceramic plate material consisting of BN of 30 to 50 wt %, Si3N4 of 30 to 65 wt % and AlN of 5 to 15 wt %.
This plate material is a material wherein, to improve the wear resistance of BN, Si3N4, which has the advantages of withstanding a long-time casting because of excellent wear resistance and small thermal expansion and also having a low cost, and further AlN, which does not have very excellent thermal shock resistance but has good wear resistance and excellent erosion resistance, are combined with BN at the rate of 30 to 65 wt % and 5 to 15 wt %, respectively. Therefore, the plate material has moderate thermal shock resistance, wear resistance (it is preferable that the wear resistance of the plate material be moderately lower than the wear resistance of the drums) and erosion resistance, and thus demonstrates the performance desirable for side dams.
Further, Japanese Unexamined Patent Publication No. H7-68354 discloses a ceramic plate material consisting of BN of 20 to 30 wt %, Si3N4 of 55 to 77 wt % and AlN of 3 to 15 wt %. This plate material also contains AlN of 3 to 15 wt % and thus has excellent erosion resistance.
However, when side dams made out of a conventional ceramic plate material are used in the continuous casting of molten stainless steel, the erosion by the chemical reaction of alloy components in the molten steel and the mechanical erosion by the molten steel in the state of semi-solidification coact, and therefore the side dams erode radically at the portions contacting the portions where the cooling drums and the molten stainless steel contact with each other. As this kink of the erosion looks like line, the erosion may be defined as linear-wear. If the erosion of the side dams is excessive as stated above, the side dams have to be changed frequently and the operation efficiency of the caster deteriorates accordingly.
Further, since the side dams used for the continuous casting of molten stainless steel also play the role of supporting molten steel containing several kinds of alloy components in large amounts and having a large specific gravity at both sides of the cooling drums for a long time, the side dams are required to be superior than the conventional ceramic plate materials not only in erosion resistance but also in basic properties.